The patterns of the interaction between a shock wave and a sonic line in transonic gas flow are studied. Using the theory of self-similar solutions, examples are constructed of the reflection of the shock wave by a sonic line in the form of a different shock wave, where the point of reflection could be a point of inflexion or cusp of the sonic line. In addition, non-self-similar flow with a triple configuration of shock waves near a sonic line for Mach reflection of the shock wave by a solid wall is constructed. (C) 1997 Elsevier Science Ltd.